<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="-" onclick="f1(1)">
<input type="button" value="+" onclick="f1(2)">
<input type="button" value="*" onclick="f1(3)">
<input type="button" value="/" onclick="f1(4)">
<div>
  结果：<span></span>
</div>
<script>
     function f1(a){
       let i1 = document.querySelector("#i1");
       let i2 = document.querySelector("#i2");
        let s = document.querySelector("span");
       if(isNaN(i1.value)||isNaN(i2.value)){
         s.innerText = "输入内容有错";
         return;
       }
       switch (a){
         case 1:
           s.innerText = i1.value-i2.value;
           break
         case 2:
           s.innerText = parseFloat(i1.value)+parseFloat(i2.value);
           break
         case 3:
           s.innerText = i1.value*i2.value;
           break
         case 4:
           s.innerText = i1.value/i2.value;
       }

     }


</script>
</body>
</html>